It has already been proposed, when it is desired to digitize very fast electric signals (for example for the acquisition of transitory signals or in the design of rapid oscilloscopes), to use the fact that the electric signal is propagated with a speed that is indeed very high but finite, and that it is possible therefore to gain access to the evolution of the electric signal in very short times by sampling the various values of this signal at different points of a line of propagation at a given moment, by means notably of ultra-fast track-and-hold units.
The values sampled by the track-and-hold units can then be read (and usually converted to digital form) off line.
Such a technique is for example described in U.S. Pat. No. 5,471,162 and French Patent No. 2 764 070 and makes it possible to obtain a large dynamic range of recording (more than 10 bits) and a very large bandwidth (greater than 10 GHz).
It is understood however that by using these principles the duration of acquisition of the signal is determined by the length of the line of propagation used for the acquisition (since these two magnitudes are linked, in an invariable manner, by the speed of the electric signal in the line, namely approximately 5 nanoseconds per meter) and that the desired increase in the duration of acquisition is consequently quickly problematical.
Because of this, a proposal has been made, in order to reduce the length of the line of propagation for a given acquisition period, to introduce a delay in the triggering (consequently successive) of the various track-and-hold units placed along the propagation line. Such a solution is for example described in patent application French Patent No. 2 779 528.
These various solutions also come up against the problem of the degradation of the signal when the number of track-and-hold units used along the line of propagation increases, which is however naturally also desirable in order to increase the number of samples acquired.
The solutions used in other fields of electric signal acquisition and the technologies used in these fields however seem to be incapable of meeting these requirements specific to the line of propagation digitizers mainly because of the necessary speed of the track-and-hold units and the simultaneity of their triggering (or of the virtual simultaneity when a delay is introduced between the various track-and-hold units). Specifically, these particular conditions imply, for example, the use of fast electronic technologies such as InP, AsGa or SiGe.
Other solutions have therefore been sought to increase the number of samples acquired by digitizers using a line of propagation such as, for example, the introduction of regenerating amplifiers between track-and-hold unit blocks (see, for example, “Contribution à l'étude et à la réalisation d'un numériseur ultra large bande à haute résolution en filière HBT InP” [Contribution to the design and production of an ultra broadband digitizer with high resolution in InP HBT process], the thesis of Hassan El Aabbaoui, Mar. 30, 2007, Université des sciences et technologies de Lille). These solutions are however quite awkward to apply in practice.
It has finally been proposed to use several propagation lines and to cause the line of propagation to function in repetitive mode and to multiplex several fast analog-digital converters behind the track-and-hold units (see for example “Etude d'éléments de base et de concepts pour un numériseur à trés large bande passante et à haute résolution” [Study of basic elements and of concepts for a very large bandwidth digitizer and at high resolution], the thesis of Benoit Gorisse, Dec. 14, 2007, Université des sciences et technologies de Lille). This solution however demands the use of fast converters, the characteristics of which, in particular in terms of the dynamic range of the converted signal, are not optimal.
In this context, the invention proposes a device for digitizing an electric signal, characterized in that it comprises a line of propagation over which the signal travels, a plurality of track-and-hold units connected at distinct points of the line of propagation so that each samples the value of the signal at its connection point, an analog matrix memory comprising a line of which at least certain elements are each connected to a track-and-hold unit so as to receive the value sampled by the track-and-hold unit, and means for line-to-line shifting of the stored values, and analog-digital conversion means for converting the stored values.
The multiplicity of the track-and-hold units allows each of them to work at a reduced frequency relative to the sampling frequency required for the whole system and the use of an analog matrix memory is therefore finally well suited to receiving in parallel the samples sampled on the propagation line.
The analog-digital conversion means comprise, for example, a plurality of analog-digital converters.
According to a first solution that may be envisaged, the analog-digital converters are each connected to a column of the analog matrix memory. It is therefore possible to read and convert the recorded values in columns at a slower pace than that of the sampling.
According to another solution that may be envisaged, an analog-digital converter is associated with each element of the analog memory, which makes it possible to continuously read the recorded values. The analog-digital converters are for example incorporated into the analog memory to do this.
It is possible to provide, in this context, means for controlling the shift at a determined frequency and means for controlling the converters at a frequency equal to said determined frequency divided by the number of elements in a column in the analog memory. The analog memory (by virtue of the plurality of elements in a column) therefore makes it possible to reduce the working frequency of the converters and notably, as a consequence, to use converters with large dynamic range.
Moreover, at least one track-and-hold unit may be connected to the analog memory by means of a differential amplifier so as to alternately apply the sampled value to a first column and to a second column of the analog memory, which makes it possible to divide by two the length of the line of propagation for a given number of samples sampled.
The track-and-hold units are for example made in InP technology, particularly suited to the necessary operating speed, while the analog memory can be made in CMOS technology, by virtue notably of the relatively low working frequency of the memory allowed by the presence of several track-and-hold units as already indicated. Also provided, for example, is a number of track-and-hold units of between 10 and 100, which allows a reduction of this working frequency by more than an order of magnitude, limiting however the length of the line to prevent the degradation of the signal.